The reproductive biology and behaviour of the fish are poorly understood. However, female fish are known to carry an extraordinary number of eggs, with an individual female capable of producing up to 300 million eggs at one time, the largest number of eggs ever recorded in a vertebrate (3) (4) (5) (8). Where and when the sunfish spawns is not well known, although five possible areas have been identified in the North and South Atlantic, the North and South Pacific, and in the Indian Ocean, where there are central rotating oceanic currents, called gyres (3) (5). The newly hatched sunfish measure just 0.25 centimetres in length, and will increase in mass by over 60 million times in order to reach the size of a 3 metre adult (3) (5) (8).
The fish is thought to feed mainly on jellyfish, and its diet may also include a variety of alternative prey species including crustaceans, molluscs, squid, small fish and deepwater eel larvae (2) (4) (5) (6) (8) (9). The frequently observed ‘basking’ behaviour, where the sunfish swims on its side at the ocean surface, is thought to be linked to the deep dives which it makes periodically throughout the day, and which are most likely made in search of prey (3) (5). Basking may be a form of thermoregulation, allowing the sunfish to warm up after making forays into cooler waters; however, other explanations for this peculiar behaviour include illness, or possibly the solicitation of cleaner fish or birds, for the removal of parasites (2) (3) (5) (9). Known to harbour many parasites, at least 40 different genera have been recorded using the sunfish as a host, and crustaceans have also been found attached to the skin and gills of many individuals (3) (4) (5).